xDSL technologies are the most widespread broadband technologies. These technologies, which comprises, for example ADSL, ADSL2, ADSL2+ or VDSL2 use Telco's copper access networks to provide broadband connectivity. xDSL signals are transmitted over metallic pairs from a Central Office to customer premises as it is shown in FIG. 1.
The downstream traffic, from the network to end users is aggregated by de DSLAM (Digital Subscriber line Access Multiplexer) 1, where there are several xDSL DSLAM line cards 2, and transmitted to the CPE (Customer Premises Equipment) 4 located at customer premises 19 over a metallic pair 11. The upstream traffic, from the end user to the network is collected by the CPE 4 and transmitted over a metallic pair 11 to the DSLAM 1 which typically is located at a Central Office 17 (FIG. 1) but it can also be located out that point, in a remote node 18 (FIG. 2).
The xDSL signals share the metallic pair 11 with voice signal from the PSTN (Public Switched Telephone Network) service. In order to share the metallic pair 11 spectrum, splitters 14a and microfilters 14b are used. For each metallic pair, there is a splitter 14a at the Central Office 17 or at the Remote Node 18 (see FIG. 2) side, and there is either a splitter 14a or microfilter 14b at the customer premises side. Voice splitters 14a and microfilters 14b divide/combine the xDSL and voice signals. The voice signal is exchanged between the PSTN switch 16 at the Central Office 17, and the telephone 15 at customer premises 19. In case of FTTN (Fiber To The Node) deployments (FIG. 2), the PSTN switch 16 can be located at Central Office 17 or if it a small one, it can be located at the remote node 18.
Splitters 14a and microfilters 14b are only required in case that voice services are provided using circuit switching technology. But the voice service can also be provide by means of VoIP (Voice over IP) using packet switching technology. In case of VoIP voice services, splitters 14a and microfilters 14b are not required.
xDSL technologies provide broadband access over existing metallic (typically copper) pair access network. But there are some constraints for these technologies:                Metallic pair attenuation. This attenuation increases with the length of the pair. That means that the SNR (Signal to Noise Ratio) decreases as metallic pair length increase. So, in order to keep the BER (Bit Error Rate) below a maximum threshold, the bitrate provided by xDSL decreases as the metallic length increases.        Crosstalk, a disturbing signal which appears when there are several xDSL links over metallic pairs that share the same cable or binder. These parasitic signals appear due to capacitive and inductive coupling between adjacent pairs. Crosstalk signals increase significantly the noise level and thus reduce significantly the SNR of the received xDSL signal in the disturbed pair.        Noise: transient signals electromagnetically coupled which appears randomly and creates burst errors.        
These constraints involve that using xDSL for broadband access, the access bit rate cannot exceed a net bitrate of 6-8 Mbit/s beyond 2.5 km away from Central Office, depending on wire gauges and pair isolation.
The introduction of fiber into the local loop, replacing partially or completely the metallic pair improves significantly the xDSL performance due to the low attenuation of fiber and its electromagnetic immunity which avoids crosstalk and noise problems. That is the reason why some Telcos has deployed FTTN xDSL access networks, following the scheme shown in FIG. 2. This solution introduces the fiber into the aggregated link between the DSLAM 1 located at the Remote Node 18 and the Central Office 17. This approach allows using short metallic pairs between each DSLAM 1 port 3 and each CPE 4. So the attenuation and crosstalk levels decrease and the noise risk also decreases. The FTTN approach could permit to extend both the coverage and the bitrate of broadband access. But it is a very expensive approach because to achieve such goal, it would be necessary to deploy a lot of remote DSLAMs which is very expensive both in capital and operation, apart from other important issues like DSLAMs feeding and the location problem that can be more complex than the base stations or Node B location for mobile services. Some patents propose solutions of this kind.
Some alternative solutions can be found looking at the patents WO0245383 A2 “Apparatus for connecting digital subscriber loops to central office equipment”, CA2346573 A1 “Arrangements for connecting digital subscriber loops to central office equipment”, CA2353594 A1 “Extended distribution of ADSL signals” or US2004264683 A1 “Hybrid Access Networks and Methods”.
But these solutions entail some other problems as looking for sites to locate the Remote Nodes 18, either in the street or in buildings; managing and monitoring remotely the equipments, the DSLAMs 1, located at the Remote Nodes 18; or remote powering of a fully equipped DSLAMs 17 located out of Central Office 17 premises.